1. Field of the Invention
The present invention relates to a method for performing automatic brake control, such as anti-lock control or traction control, in an automotive vehicle. More particularly, the invention is concerned with improvements in the method for controlling a braking system, by changing a plurality of control parameters in two or more steps, depending upon a specific one of a plurality of ranges in which a current value of a given variable falls.
2. Discussion of the Prior Art
A braking system for a motor vehicle is controlled so as to automatically regulate the braking pressures for the wheels, for anti-lock control, traction control or other purposes. The anti-lock control is interpreted to mean automatic regulation of the braking forces applied to the wheel brakes, during brake application to the vehicle, so as to prevent the braking forces from exceeding upper limits, beyond which a ratio of slip of the wheels on the road surface exceeds a permissible level. On the other hand, the traction control is performed for automatically activating the wheel brakes to more or less resist drive forces of the drive wheels, upon acceleration of the vehicle, in order to prevent an excessive slip ratio of the drive wheels due to an excessive torque applied to the drive wheels.
To achieve such automatic control operations of the braking system for a motor vehicle, some control parameters are used. In an anti-lock control, for example, an upper limit of a deceleration rate of the vehicle, and a time during which the braking pressures are reduced, are used as the control parameters to regulate the braking pressures. Described in more detail, the highest one of the rotating speeds of all wheels is presumed to represent the actual running speed of the vehicle. When the deceleration rate of the presumed speed of the vehicle exceeds a preset upper limit, the deceleration rate is fixed at that preset upper limit, and the running speed of the vehicle is presumed based on that upper limit. In this way, the upper limit of the deceleration rate is used as a control parameter. In the meantime, when the rotating speed of a given wheel deviates from the presumed running speed of the vehicle by a predetermined amount, the braking pressure for that wheel is reduced for a suitable length of time. Thus, the pressure-reducing time between the start and the end of the pressure reduction is used as another control parameter. Usually, these control parameters are changed depending upon a change of a given variable such as a rotating speed of the wheel, a rate of acceleration or deceleration of the vehicle, or a time lapse after a control cycle is started. For economical control arrangement, it is desirable that the control parameters are changed in two or more steps which correspond to a plurality of ranges of the selected variable. Namely, the parameters are changed depending upon a specific one of the ranges in which the currently detected variable falls.
The foregoing situations apply to the traction control, or other automatic control operations of the braking system.
In an effort to accomplish an optimum anti-lock control operation of an anti-lock braking system, the present inventors attempted to use a deceleration rate of a rotating speed of each wheel as a variable, and the above-indicated upper limit of the deceleration rate and pressure-reducing time as control parameters. To this end, the vehicle is provided with a deceleration sensor for detecting an actual rate of deceleration of the vehicle. Further, three ranges (first, second and third ranges) are established for the detected deceleration rate G, so that the detected rate falls within one of the three ranges. The upper limit of the deceleration rate and the pressure-reducing time, as the control parameters, are changed in three steps which correspond to the three ranges of the detected deceleration rate G, as indicated in TABLE 1.
TABLE 1 ______________________________________ Detected Deceleration Upper Limit Pressure Range Rate G Of G Reducing Time ______________________________________ First -0.5 g &gt; G -1.15 g 20 msec (short) Second -0.2 g &gt; G .gtoreq. -0.5 g -0.8 g 50 msec (middle) Third G .gtoreq. -0.2 g -0.5 g 80 msec (long) ______________________________________
The above method of control in which the upper limit of the deceleration rate and the pressure-reducing time are changed in three steps, permits a generally high degree of accuracy in the anti-lock control operation of the braking system. However, the inventors discovered that the above arrangement suffers from an impermissible control error in some special cases, for example, where the coefficient of friction of the road surface suddenly changes during the anti-lock control operation, or where an error arises in the measurement of the vehicle deceleration rate by the deceleration sensor.
An investigation conducted by the inventors reveals that in such special cases, an erroneous determination is made of the range in which the detected variable (deceleration rate) falls, and the control parameters are consequently set to the inappropriate values, based on the erroneous determination. In this connection, it was found that the use of the inappropriate control parameters which deviate from the optimum values is more or less impermissible in some cases, but more or less permissible in the other cases. For instance, if the upper limit of the deceleration rate is erroneously set to -1.15 g while it must be set to -0.8 g, this error does not cause an impermissible result. Conversely, if the upper limit is erroneously set to -0.8 g while it must be set to -1.15 g, this error results in an impermissible consequence, i.e., unduly increased braking distance.